The present invention relates to a method for forming a structure composed of a graphite/epoxy composite material and more particularly to a method for conforming such composites precisely to the contour of a mold.
Aircraft components are currently being manufactured from graphite/epoxy composite materials. These composite materials must be very precisely formed when used as aerodynamic surfaces, especially wing surfaces, in order to meet critical aerodynamic tolerances that are required to reduce drag.
Fabrication of graphite/epoxy composites in many instances requires that precured or partially precured skins or other sheets of the composite material be combined with additional graphite/epoxy composite material and thereafter cured. Such composite structures, especially components that are intended to function as surface panels, are held with pressure against a mold. A typical mold is constructed of aluminum, steel or a fiberglass/epoxy composite; however, these materials have significantly different thermal expansion characteristics than does the graphite/epoxy material. When the graphite/epoxy material and an aluminum mold, for example, are heated under pressure, the aluminum mold expands faster than the precured skin. Sufficient friction exists between the skin and the mold surface to prevent relative movement between the two. This lack of relative movement induces strain in the precured skin during the curing cycle and causes the composite material to warp when cooled down and removed from the mold. As a consequence, the warped part is unacceptable as it cannot meet the critical tolerances required for aerodynamic surfaces.
Prior art attempts to solve the warping problem included post-aging of the part while forcing it to the desired contour. Another attempt required remachining of the mold to a contour that would, in essence, compensate for the stresses induced in the precured skin so that after it was cured, cooled, and removed from the mold, it would warp into the desired shape. These attempted solutions proved unsatisfactory, however, as they were either unsuccessful or added unnecessary expense.